The moment Microsoft announced it would lock other browsers out of being installed on Windows RT, we all knew regulatory bodies the world over were wringing their hands. Today, this has been confirmed: in the wake of an investigation into Microsoft not complying with the existing antitrust rulings regarding browser choice, the EU has also announced it's investigating Windows 8 x86 and Windows 8 RT (ARM).

Microsoft is not going to allow any other browser to be installed on Windows RT. Huge difference.

As for email, there is the excellent Sparrow email client. The one thing Apple does not allow, and I agree this is wrong, is that you cannot change the setting for the default client for email and web browsing.

iOS mandates that 3rd-party browsers use the webkit engine that comes with iOS, though. So Firefox is banned,because Firefox uses gecko. Only if Mozilla changed Firefox to use the iOS webkit engine, would Apple allow Firefox to run on iOS.

Microsoft isn't nearly as harsh in that respect. Microsoft will allow Firefox to run on WindowsRT, but Mozilla says that their javascript engine won't run fast because WindowsRT lacks access to certain Win32 apis (WinRT only supports a subset of Win32 api).
So Microsoft is already treating Mozilla better than Apple does. Moreover, just as Mozilla could change Firefox to use webkit when running on iOS, Mozilla could change Firefox to use the IE engine (I think it's called "Trident"), when running on WindowsRT.

So WindowsRT is at least as permissive as iOS wrt Mozilla, and actually more so. Mozilla is being hypocritical by complaining about WindowsRT (which as zero percent marketshare, BTW) while not lodging similar complaints against iOS, which has a harsher policy AND controls 80% of the ARM market.

Microsoft is not going to allow any other browser to be installed on Windows RT.

Not at present, anyway. But Apple didn't allow any other browser than Safari on the iPad and iPhone initially, as well. There will undoubtedly be pressure on Microsoft to allow other browsers and, like Apple, they will probably relent, since they have corporate customers that use other browsers.

There are several different browsers you can install on an iPad and use besides Safari.... Google Chrome, Dolphin, just to name two, check out this list: http://appadvice.com/appguides/show/ipad-web-browsers
Microsoft is not going to allow any other browser to be installed on Windows RT. Huge difference.

Those are just UIs to iOS-included Webkit engine, basically the same as used by Safari, but with worse javascript engine.

Essentially just what MS wants to do with RT ...which is in a way less restrictive: you can use your own HTML parser (but not js engine, performance of which will hence also suffer)